This invention pertains to fire resistant composition boards of the class relying upon the inclusion of a non-combustible mineral substance for its fire resisting properties. It pertains particularly to fire resistant composition boards containing vermiculite.
In a widely practiced commercial process for making composition board, a fibrous cellulosic pulp slurry is run onto a Fourdrinier wire screen and vacuum formed into a wet sheet. The sheet is roller pressed to the desired thickness and kiln dried.
Fire resistant composition board is made similarly, employing a substantial proportion of a mineral substance in the slurry. Mineral substances thus employed include gypsum, perlite, mineral fiber fiberglass and vermiculite.
Gypsum board is widely used, but is heavy and lacks structural strength.
Perlite board shrinks and cracks when exposed to fire. This creates openings which permit transmission and spreading of the fire.
Mineral fiber board also shrinks materially when heated.
Fiberglass board sinters and melts down when exposed to fire, with the same adverse result.
Vermiculite board, as known to the prior art, has inherent disadvantages of a character to be developed in the ensuing discussion.
Vermiculite is a gold-colored mineral substance of widespread occurrence which is available at competitive cost. Its formula is as follows: EQU 3 MgO(FeAl).sub.2 O.sub.3, 3SiO.sub.2
vermiculite as quarried has a certain content of included water. When heated, the water is converted to steam which, as it is released, expands the vermiculite. This transforms the vermiculite to an expanded product of relatively low density. The expanded product still retains, of course, the inherent noncombustible qualities of the parent mineral.
Heat-expanded vermiculite is readily available commercially. It is manufactured throughout the world by crushing vermiculite ore, screening the crushed product to specific particle sizes, and expanding the classified material by flash heating it in a furnace at about 1800.degree. F. The expanded product is screened again, distributed, and applied to its many commercial uses.
In many respects the expanded vermiculite of commerce is well suited for inclusion in fire resistant composition board and like products. It is light in weight, has good properties of fire resistance, can be made into board products using conventional equipment, enhances the appearance of the board, and is available in large quantities at competitive cost.
However, expanded vermiculite in particle form imparts poor drainage qualities to aqueous slurries in which it is included with the result that the slurries form into sheets slowly when run onto the Fourdrinier wire. Also, the finer vermiculite particles, as well as the other finely divided solid components of the slurry, tend not to be retained in the sheet, but to run off with the drainings. This causes a loss of vermiculite and the other solid components with resultant problems of waste disposal.
I now have discovered that the first named of the foregoing problems, namely the poor drainage properties of board-making slurries containing vermiculite, may be overcome by the inclusion in the slurries of a proportion of a particular high-freeness defiberized wood additive. I also have discovered that the second of the problems noted above, namely the failure of the vermiculite to be retained in the sheet on the forming wire, may be overcome by including in the board-making furnish a proportion of hydrated cellulosic gel.
Still further, I have discovered that if the vermiculite is employed in partially or incompletely expanded form it overcomes one of the primary problems attendant upon the use of mineral-containing fire resistant composition boards: their tendency to transmit a fire because of sintering, collapsing, or cracking of the boards when exposed to high temperatures. In the practice of the present invention, advantage is taken of the inherent property of incompletely expanded vermiculite to expand further when heated. This expansion occurs between 1000.degree. and 1800.degree. F., while the heat developed by a fire, and in particular the temperature stipulated in Underwriters' Fire Tests for fire resistant composition board, is of the order of 2200.degree. F.
Accordingly, when incompletely expanded vermiculite is used in the manufacture of a fire resistant composition board, and the board thereafter is exposed to fire, the vermiculite expands further. This causes the board to thrust outwardly in its own plane against adjacent panels, or against frames such as fire door frames in which it is contained. This exerts a sealing pressure and for a substantial time inhibits or prevents the formation of cracks and voids which would transmit the fire.
Another difficulty, which is shared by vermiculite with other minerals employed in the manufacture of fire resistant composition board, is that caused by the combustion of the starch or other organic binders conventionally employed with the mineral substances in the fabrication of the board. In a fire, the board collapses upon burning of the organic binder.
I also have discovered that by the inclusion in a vermiculite board-making slurry of a proportion of a ceramic clay having a sintering temperature below the temperature of burning organic materials, there is provided a backup binder which in a fire situation is available to insure the dimensional stability and integrity of the board after the organic binders have been consumed. Thus, if the clay sinters at about 1200.degree. F. and the temperature developed by a conflagration is about 2200.degree. F., the sintered clay on the board surface will serve as a binder which is developed before the organic binders burn away entirely. thereby holding the board together.
Yet another problem with vermiculite fireproof composition board, which is shared also by boards containing fire resistant minerals of other classes, resides in the fact that when exposed to fire the cellulose, starch and other organic constituents of the board are subjected to high temperatures at which they decompose into combustible gases. Upon escaping, these gases are ignited and help to spread the fire.
I have discovered further thay by coating at least one face of the fire resistant vermiculite composition board of my invention with a substantially continuous and impervious layer of sodium silicate, the layer thus applied serves as a seal which for an appreciable time seals off the surface of the board, preventing the escape into the fire environment of combustible gaseous degradation products of the organic components of the board and thus preventing their contribution of the spreading of the fire.